The present invention relates to a plant for breeding, farming and storage of fish and other marine species of the kind comprising a net cage which is installed in a submerged position in the sea. The invention is particularly adapted for installation in open or offshore environments as distinct from sheltered and shallow environments.
Fish farming plants of this kind consist basically of a plurality of anchored floating bodies which support a net cage downwardly suspended in the sea. In order to stabilize the net cage in a substantially vertically expanded configuration in the sea, weights and/or bottom anchorings are arranged along the bottom edge of the net cage. In connection with fish farming plants to be installed in sheltered waters, these are often equipped with suitable platforms supplied with gangways, and suspended fish feeding means.
Known plants for farming and storing or caging fish in the sea have, on account of their shape and equipment, a limited field of utilization due to the prevailing environmental forces, water depths etc. The installation site of the plants creates problems such as a risk for local contamination and traffic impediments. Consequently, both fish farmers and authorities want to develop plants which can be positioned in more open, deep water areas. Location of the plants in open deep waters will also render possibilities for installation of larger plants.
In recent years, several types of fish farming plants have been developed, designed for use in offshore environments. As illustrative for the prior art in this field reference made be made to U.S. Pat. No. 4,957,064, Swedish Patent No. 450866 and Norwegian Patent No. 153991.
The U.S. patent shows a fish cage structure consisting of a cagelike enclosure which is provided with circumferentially arranged float means at the top, designed to float in the sea surface. The net cage is upwardly open and must accordingly not be pulled down below the sea surface, otherwise the fish would escape. Along the bottom edge of the fish cage, weights are provided in order to tension the cage downwards. Instead of weights, the bottom edge of the cage may be provided with anchor lines to the sea bottom. On account of the interconnected, complicated floating system shown, the plant will be severely exposed to lateral forces from wind and waves.
Swedish patent no. 450866 shows a fish farming plant where the net cage consists of a lattice framework assembled from rods which are linked together in the nodes. The framework circumscribes and supports a net cage with a closed ceiling. The plant is designed for use in open waters, i.e. an "offshore" fish farming plant. The plant seems to imply two substantial drawbacks. Firstly, such a relatively tight or compact construction of the plant will be severely affected by lateral forces, i.e. environmental forces from waves, wind, stream etc. Furthermore, maintenance of the plant will be complicated, since the cage is permanently attached to the framework at a plurality of circumferentially distributed points. Growth or fouling might also represent a problem.
Norwegian patent 153991 shows a plant where a submerged cage is used, i.e. a cage which is normally positioned below the sea surface. At the installation site, the cage is pulled down to the selected level. At the top, the cage is provided with ventilation and illumination equipment. A plant with a permanently submerged fish cage will require special air supply means if the plant is to be used for breeding or farming fish belonging to the salmon family.